Precision winding of coils in which the turns of each successive layer are laid or nested in the valleys between the close-wound turns of the preceding layer is desired because it is most compact and therefore most economical of copper or aluminum wire for the coil and of iron for the core. In coil winding, the wire may be wound on the bobbin by revolving the bobbin as the wire is guided to it, or, alternatively, by wrapping the wire around a stationary bobbin by means of a revolving winding head, commonly referred to as a fly head winder. In either method, for compact precision winding, the wire guide or the fly head is caused to traverse across the width of the bobbin at a constant rate determined by the width of the wire and corresponding to one wire diameter per turn.
A constant rate of traverse comparable to the winding of a helix assures smooth even coiling when winding on a round drum-like surface but not when winding on a rectangular or box-like surface. The laminated iron cores used in transformers and reactors are made by stacking laminae so that they are rectangular in cross section. The bobbins are ordinarily provided with rectangular windows to accommodate the cores, and as a result, the coil support surface or drum is rectangular in cross section. In such a rectangular bobbin, an inclined wedge, sometimes known as a kicker, is built in next to the flange where winding starts. The kicker is put in the fourth panel of the winding surface and, starting with zero width, widens to one wire diameter over the length of the panel. Its function is to force the wire over so that it lies alongside the first turn at the start of the second turn.
In coiling, in order to assure tight close-wound turns, the wire is supplied to the bobbin at a slight angle off normal to the drum surface, so that it "leans" slightly, initially into the flange, and thereafter into the preceding turn. A uniform rate of traverse of the wire guide or fly head winder when winding on a rectangular bobbin entails a cyclic variation in the extent to which the wire leans against the preceding turn. Starting at the flange, on the first, second, and third panels of the bobbin's winding surface, the wire leans progressively less against the flange and as it is put down parallel to the flange. On the fourth panel, the kicker forces the wire laterally over at a slight diagonal away from the flange, so that it is displaced the width of one turn or wire diameter before encountering the first panel again. This restores the "lean" to its maximum. Upon reaching the fourth panel on the next turn, it is the prelaid inclined wire of the previous turn that forces the wire over, and this happens for each succeeding turn in the layer. Thus in precision winding according to the current practice, the wire is continuously supplied at a cyclically varying "lean" or pressure against the prelaid turn and must slide down and off the shoulder of the prelaid turn to lie alongside of it. The new turns will conform to the wire lay only so long as the foregoing takes place.
The "lean" or pressure against the prelaid turn varies cyclically from a maximum at the beginning of the first panel to something not less than zero at the beginning of the fourth panel. There cannot be a negative pressure, but if the wire does not lean against the preceding turn at all, there may be a gap between turns which, if cumulated, will cause a defect. At the other extreme, if the "lean" or pressure is too great, the wire will not slide off and fall alongside the preceding turn but will, instead, climb up on the preceding turn, thus creating another kind of defect. On the next layer of turns, the defect may be repeated and magnified. At each layer of turns, the direction of the "lean" must be reversed. Tolerance errors such as those due to undersize or oversize wire are cumulative at least through a layer so that defects tend to occur at the reversals or just prior to the reversals.
Precision winding by the above-described prior art method of conforming to the prelaid wire lay has required stringent control of wire size and ductility, the use of accurately formed bobbins free of defects, and careful machine adjustment including control of wire tension. The extent to which tension in the wire causes the wire to stretch during winding varies with ductility of the wire and stretching effectively reduces the wire size so that control of wire tension may be critical. Since the errors in tolerance are cumulative throughout a layer, the machine must be closely watched by an operator to make sure that the lay down of wire is good.